Distributed messaging system supporting stateful subscriptions

ABSTRACT

A distributed messaging system supporting stateful subscriptions is provided. A stateful publish-subscribe system extends the functionality of the content-based approach to include more general state-valued expressions. Stateful subscriptions may refer to one or more message histories and may include more complex expressions. Therefore, subscribers may receive different information than that provided in the published messages. A plurality of broker machines is provided to deliver messages sent by publishing clients toward subscribing clients based upon the contents of the messages and stateful transformations requested by the subscribing clients. Subscription specifications are analyzed by a compiler and converted into a collection of transform objects and view objects. The messaging system builds a structure containing all transform objects and view objects needed for all intermediate and subscribed views of all subscriptions. This messaging system uses this structure to allocate transform objects and view objects to broker machines in the overlay network.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No. 10/841,297, entitled “CONTINUOUS FEEDBACK-CONTROLLED DEPLOYMENT OF MESSAGE TRANSFORMS IN A DISTRIBUTED MESSAGING SYSTEM,” filed on an even date herewith, status abandoned; assigned to the same assignee, and incorporated herein by reference.

This application is a continuation of application Ser. No. 10/841,916, filed May 7, 2004, now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to data processing systems and, in particular, to messaging systems in a distributed processing environment. Still more particularly, the present invention provides a distributed messaging system supporting stateful subscriptions.

2. Description of Related Art

A publish-subscribe messaging middleware is a system in which there are two types of clients. Publishers generate messages, also referred to as events, containing a topic and some data content. Subscribers request a criterion, also called a subscription, specifying what kind of information, based on published messages, the system is to deliver in the future. Publishers and subscribers are anonymous, meaning that publishers do not necessarily know how many subscribers there are or where they are and, similarly, subscribers do not necessarily know where publishers are.

A topic-based, or content-based, publish-subscribe system is one in which the delivered messages are a possibly filtered subset of the published messages and the subscription criterion is a property that can be tested on each message independent of any other message. For example, a filter may determine whether “topic=stock-ticker” or “volume>1000.” Content-based or topic-based publish-subscribe systems are referred to herein as “stateless.”

There are pre-existing and emerging alternative technologies to solve the deficiencies of content-based publish-subscribe systems. Message mediators may be introduced into the flow of traditional messaging middleware. This is a useful concept; however, in their current manifestations, mediators are complex to program, require external database services in order to store and access state, and groups of mediators cannot be automatically combined.

Traditional database systems may also be used. Each published message can give rise to a cascade of transactions updating the message history. Subscriptions can be expressed as views of these histories. Technologies are being developed to allow views to be updated incrementally. Such an approach is easier to program; however, it can be costly and slow if each new message results in a transaction involving a large number of subscribers.

An emerging technology still being researched is continuous queries on data streams. These approaches preserve the simpler programming model of the database system approach above and attempt to reduce the cost of traditional databases by a combination of approaches, including batching message updates and restricting the available operations to ones allowing the use of bounded-sized, in-memory sliding windows. However, this approach is restricting and limited.

SUMMARY OF THE INVENTION

The present invention solves the disadvantages of the prior art and provides a distributed messaging system supporting stateful subscriptions. A stateful publish-subscribe system extends the functionality of the content-based approach to include more general state-valued expressions. Stateful subscriptions may refer to one or more message histories and may include more complex expressions. Therefore, subscribers may receive different information than that provided in the published messages. A plurality of broker machines is provided to deliver messages sent by publishing clients toward subscribing clients based upon the contents of the messages and stateful transformations requested by the subscribing clients. These broker machines form an overlay network. Subscription specifications are analyzed by a compiler and converted into a collection of transform objects and view objects. The messaging system builds a structure containing all transform objects and view objects needed for all intermediate and subscribed views of all subscriptions. This messaging system uses this structure to allocate transform objects and view objects to broker machines in the overlay network.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

FIG. 1 depicts a pictorial representation of a network of data processing systems in which the present invention may be implemented;

FIG. 2 is a block diagram of a data processing system that may be implemented as a server in accordance with a preferred embodiment of the present invention;

FIG. 3 is a block diagram of a data processing that may serve as a client of a service in accordance with a preferred embodiment of the present invention;

FIG. 4 illustrates a broker network for a publish-subscribe system in accordance with a preferred embodiment of the present invention;

FIG. 5 illustrates how a stateful publish-subscribe service of the present invention appears to clients;

FIG. 6 illustrates an example of a operator that transforms input view objects to an output view object in accordance with a preferred embodiment of the present invention;

FIG. 7 illustrates an example dataflow hypergraph distributed over multiple brokers in accordance with a preferred embodiment of the present invention;

FIG. 8 depicts a process for deploying transform objects and view objects when a dataflow specification is a declarative specification in accordance with a preferred embodiment of the present invention;

FIG. 9 is a flowchart illustrating the configuration and deployment of a stateful publish-subscribe system in accordance with a preferred embodiment of the present invention; and

FIG. 10 is a flowchart illustrating the operation of a stateful publish-subscribe system at runtime in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method, apparatus and computer program product for supporting stateful subscriptions in a distributed messaging system. The data processing device may be a stand-alone computing device or may be a distributed data processing system in which multiple computing devices are utilized to perform various aspects of the present invention. Therefore, the following FIGS. 1-3 are provided as exemplary diagrams of data processing environments in which the present invention may be implemented. It should be appreciated that FIGS. 1-3 are only exemplary and are not intended to assert or imply any limitation with regard to the environments in which the present invention may be implemented. Many modifications to the depicted environments may be made without departing from the spirit and scope of the present invention.

With reference now to the figures, FIG. 1 depicts a pictorial representation of a network of data processing systems in which the present invention may be implemented. Network data processing system 100 is a network of computers in which the present invention may be implemented. Network data processing system 100 contains a network 102, which is the medium used to provide communications links between various devices and computers connected together within network data processing system 100. Network 102 may include connections, such as wire, wireless communication links, or fiber optic cables.

In the depicted example, servers 112, 114, 116 are connected to network 102 along with storage unit 106. In addition, clients 122, 124, and 126 are connected to network 102. These clients 122, 124, and 126 may be, for example, personal computers or network computers. In the depicted example, servers 112, 114, 116 provide data, such as boot files, operating system images, and applications to clients 122, 124, 126. Clients 122, 124, and 126 are clients to servers 112, 114, 116. Network data processing system 100 may include additional servers, clients, and other devices not shown.

In accordance with a preferred embodiment of the present invention, network data processing system 100 provides a distributed messaging system that supports stateful subscriptions. A subset of clients 122, 124, 126 may be publishing clients, while others of clients 122, 124, 126 may be subscribing clients, for example. Published events may also be generated by one or more of servers 112, 114, 116.

A stateful publish-subscribe system is a distributed messaging system in which at least one subscription is stateful. Other subscriptions may be content-based or, in other words, stateless. In other words, a stateful publish-subscribe system must compute information that requires multiple messages of one or more streams. For example, a stateful subscription may request, “Give me the highest quote within each one-minute period.” A stateful subscription may entail delivering information other than simply a copy of the published messages. For example, a stateful subscription may request, “Tell me how many stocks fell during each one-minute period.”

The stateful publish-subscribe system is implemented within an overlay network, which is a collection of service machines, referred to as brokers, that accept messages from publisher clients, deliver subscribed information to subscriber clients, and route information between publishers and subscribers. One or more of servers 112, 114, 116, for example, may be broker machines.

Both content-based and stateful publish-subscribe systems support a message delivery model based on two roles: (1) publishers produce information in the form of streams of structured messages; and, (2) subscribers specify in advance what kinds of information in which they are interested. As messages are later published, relevant information is delivered in a timely fashion to subscribers.

Content-based subscriptions are restricted to Boolean filter predicates that can only refer to fields in individual messages. For example, a content-based subscription may request, “Deliver message if traded volume>1000 shares.” On the other hand, stateful subscriptions are more general state-valued expressions and may refer to one or more messages, either by referring to multiple messages of a single message stream or by referring to multiple message streams or both. In a content-based publish-subscribe system, because subscriptions can only specify filtering, all published messages are either passed through to subscribers or filtered out. Therefore, messages received by subscribers are identically structured copies of messages published by publishers. In contrast, in a stateful publish-subscribe system, subscriptions may include more complex expressions and, therefore, subscribers may receive information that is not identical to the published messages with different formatting. For example, a published message may have only integer prices, while subscriptions to average prices may have non-integer averages.

Published message streams are associated with topics. Each topic is associated with a base relation. A base relation is a table of tuples, each tuple corresponding to an event in the particular message stream. Subscriptions are expressed as view expressions in a relational algebraic language, although other representations may be used, such as eXtensible Markup Language (XML), for example. The language defines a cascade of views of base relations and derived views computed from either base relations or other views. At compile-time, the set of subscriptions is compiled into a collection of objects that are deployed and integrated into messaging brokers. At run-time, publishers and subscribers connect to these brokers. Published events are delivered to objects associated with base relations. The events are then pushed downstream to other objects that compute how each derived view changes based on the change to the base relation. Those derived views associated with subscriptions then deliver events to the subscriber informing the subscriber of each change in state.

In the depicted example, network data processing system 100 is the Internet with network 102 representing a worldwide collection of networks and gateways that use the Transmission Control Protocol/Internet Protocol (TCP/IP) suite of protocols to communicate with one another. At the heart of the Internet is a backbone of high-speed data communication lines between major nodes or host computers, consisting of thousands of commercial, government, educational and other computer systems that route data and messages. Of course, network data processing system 100 also may be implemented as a number of different types of networks, such as for example, an intranet, a local area network (LAN), or a wide area network (WAN). FIG. 1 is intended as an example, and not as an architectural limitation for the present invention.

Referring to FIG. 2, a block diagram of a data processing system that may be implemented as a server, such as server 104 in FIG. 1, is depicted in accordance with a preferred embodiment of the present invention. Data processing system 200 may be a symmetric multiprocessor (SMP) system including a plurality of processors 202 and 204 connected to system bus 206. Alternatively, a single processor system may be employed. Also connected to system bus 206 is memory controller/cache 208, which provides an interface to local memory 209. I/O bus bridge 210 is connected to system bus 206 and provides an interface to I/O bus 212. Memory controller/cache 208 and I/O bus bridge 210 may be integrated as depicted.

Peripheral component interconnect (PCI) bus bridge 214 connected to I/O bus 212 provides an interface to PCI local bus 216. A number of modems may be connected to PCI local bus 216. Typical PCI bus implementations will support four PCI expansion slots or add-in connectors. Communications links to clients 108-112 in FIG. 1 may be provided through modem 218 and network adapter 220 connected to PCI local bus 216 through add-in connectors.

Additional PCI bus bridges 222 and 224 provide interfaces for additional PCI local buses 226 and 228, from which additional modems or network adapters may be supported. In this manner, data processing system 200 allows connections to multiple network computers. A memory-mapped graphics adapter 230 and hard disk 232 may also be connected to I/O bus 212 as depicted, either directly or indirectly.

Those of ordinary skill in the art will appreciate that the hardware depicted in FIG. 2 may vary. For example, other peripheral devices, such as optical disk drives and the like, also may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural limitations with respect to the present invention. The data processing system depicted in FIG. 2 may be, for example, an IBM eServer™ pSeries® system, a product of International Business Machines Corporation in Armonk, N.Y., running the Advanced Interactive Executive (AIX) operating system or LINUX operating system. An object oriented programming system such as Java may run in conjunction with the operating system and provides calls to the operating system from Java programs or applications executing on data processing system 300. “JAVA” is a trademark of Sun Microsystems, Inc.

With reference now to FIG. 3, a block diagram of a data processing that may serve as a client of a service in accordance with a preferred embodiment of the present invention. Data processing system 300 is an example of a computer, such as client 108 in FIG. 1, in which code or instructions implementing the processes of the present invention may be located. In the depicted example, data processing system 300 employs a hub architecture including a north bridge and memory controller hub (MCH) 308 and a south bridge and input/output (I/O) controller hub (ICH) 310. Processor 302, main memory 304, and graphics processor 318 are connected to MCH 308. Graphics processor 318 may be connected to the MCH through an accelerated graphics port (AGP), for example.

In the depicted example, local area network (LAN) adapter 312, audio adapter 316, keyboard and mouse adapter 320, modem 322, read only memory (ROM) 324, hard disk drive (HDD) 326, CD-ROM driver 330, universal serial bus (USB) ports and other communications ports 332, and PCI/PCIe devices 334 may be connected to ICH 310. PCI/PCIe devices may include, for example, Ethernet adapters, add-in cards, PC cards for notebook computers, etc. PCI uses a cardbus controller, while PCIe does not. ROM 324 may be, for example, a flash binary input/output system (BIOS). Hard disk drive 326 and CD-ROM drive 330 may use, for example, an integrated drive electronics (IDE) or serial advanced technology attachment (SATA) interface. A super I/O (SIO) device 336 may be connected to ICH 310.

An operating system runs on processor 302 and is used to coordinate and provide control of various components within data processing system 300 in FIG. 3. The operating system may be a commercially available operating system such as Windows XP™, which is available from Microsoft Corporation. An object oriented programming system such as Java may run in conjunction with the operating system and provides calls to the operating system from Java programs or applications executing on data processing system 300. “JAVA” is a trademark of Sun Microsystems, Inc. Instructions for the operating system, the object-oriented programming system, and applications or programs are located on storage devices, such as hard disk drive 326, and may be loaded into main memory 304 for execution by processor 302. The processes of the present invention are performed by processor 302 using computer implemented instructions, which may be located in a memory such as, for example, main memory 304, memory 324, or in one or more peripheral devices 326 and 330.

Those of ordinary skill in the art will appreciate that the hardware in FIG. 3 may vary depending on the implementation. Other internal hardware or peripheral devices, such as flash memory, equivalent non-volatile memory, or optical disk drives and the like, may be used in addition to or in place of the hardware depicted in FIG. 3. Also, the processes of the present invention may be applied to a multiprocessor data processing system.

For example, data processing system 300 may be a personal digital assistant (PDA), which is configured with flash memory to provide non-volatile memory for storing operating system files and/or user-generated data. The depicted example in FIG. 3 and above-described examples are not meant to imply architectural limitations. For example, data processing system 300 also may be a tablet computer, laptop computer, or telephone device in addition to taking the form of a PDA.

In accordance with a preferred embodiment of the present invention, a plurality of broker machines are responsible for delivery of message sent by publishing clients towards subscribing clients based upon the content of the messages and the stateful transformations requested by the subscribing clients. These broker machines form an overlay network. Some broker machines may be specialized for hosting publishing clients, referred to as publisher hosting brokers (PHB), and others for hosting subscribing clients, referred to as subscriber hosting brokers (SHB). Between the PHBs and the SHBs, there may be any number of intermediate nodes that include routing and filtering. The brokers at the intermediate nodes are referred to as intermediate brokers or IBs. For expository purposes, this separation is assumed; however, in actual deployment, some or all of the broker machines may combine the functions of PHB, SHB, and/or IB.

FIG. 4 illustrates a broker network for a publish-subscribe system in accordance with a preferred embodiment of the present invention. A publishing client, such as one of publishers 402 a-402 d, establishes a connection to a PHB, such as PHB 404 a or PHB 404 b, over a corresponding one of client connections 406 a-406 d. The client connection may be, for example, any reliable first-in/first-out (FIFO) connection, such as a Transmission Control Protocol/Internet Protocol (TCP/IP) socket connection. Independently, a subscribing client, such as one of subscribers 412 a-412 d, establishes a connection to a SHB, such as SHB 410 a or SHB 410 b, over a corresponding one of client connections 414 a-414 d, which may be similar to client connections 406 a-406 d. The PHBs and SHBs are connected, via intermediate brokers 408 a-408 b, through broker-to-broker links.

The publish-subscribe system of the present invention may include a fault-tolerant protocol that tolerates link failures and message re-orderings, in which case it is not necessary for the broker-to-broker connections to use reliable FIFO protocols, such as TCP/IP, but may advantageously use faster, less reliable protocols. Each broker machine may be a stand-alone computer, a process within a computer, or, to minimize delay due to failures, a cluster of redundant processes within multiple computers. Similarly, the links may be simple socket connections, or connection bundles that use multiple alternative paths for high availability and load balancing.

In the example depicted in FIG. 4, one or more execution engine may run on the brokers in the broker network and be interconnected. The one or more execution engines may be interconnected to form a distributed execution engine. The execution engines running on the plurality of broker machines receive input messages, process the input messages using the transform objects, and route output messages toward subscribers.

FIG. 5 illustrates how a stateful publish-subscribe service of the present invention appears to clients. Clients are unaware of the physical broker network or its topology. A client application may connect to any broker in the role of publisher and/or subscriber. Publishing clients are aware only of particular named published message streams, such as message streams 502, 504. Multiple clients may publish to the same message stream.

Administrators and clients may defined derived views based on functions of either published message streams or of other derived views. In the depicted example, message streams may be represented as relations. Derived views are represented as relations derived from published message streams or from other derived views by means of relational algebraic expressions in a language, such as Date and Darwen's Tutorial-D, Structured Query Language (SQL), or XQUERY. For example, derived view 510 is defined as a function of stream relations 502 and 504 by means of a JOIN expression with relations 502 and 504 as inputs and relation 510 as an output. Similarly, relation 512, indicated as a subscriber view, is derived from relation 510 by client-specified relational expressions. For example, subscriber view 512 may be a request to group the stock trades of relation 510 by issue and hour and compute the running total volume and max and min price for each issue-hour pair.

Each subscribing client subscribes to a particular derived view. As published events enter the system from publishing clients, they are saved in their respective streams. The system is then responsible for updating each derived view according to the previously specified relational expressions and then delivering client messages to each subscriber representing the changes to the state of the respective subscribed view.

In a preferred embodiment of the present invention, subscription specifications are analyzed by a compiler and converted into a collection of transform objects and view objects. Each operator that derives a view from one or more inputs corresponds to a transform object. Each view corresponds to a view object. View objects hold the state of a view. Transform objects express the logic for incrementally updating an output view constituting the result of an operator in response to individual changes to input views constituting the arguments to that operator.

FIG. 6 illustrates an example of a operator that transforms input view objects to an output view object in accordance with a preferred embodiment of the present invention. In the depicted example, views 610 and 620 are view objects that are inputs to some operator, such as, for example, a JOIN operator. Transform 650 is a transform object for that operator, which produces a derived view shown as view object 670. When one of the input objects changes, either because it itself is a published input stream or because it is a derived view that has changed as a result of changes to its inputs, messages reflecting the changes are sent to transform object 650. Transform 650 receives the messages representing changes to its inputs 610, 620, computes how the result of the operator changes given the announced changes it has received, and then delivers the computed results to its output view object 670 in the form of change messages. Output view object 670 then propagates in its turn such change messages, either to further transforms, if view object 670 is an intermediate view, or to subscribers, if view object 670 is a subscriber view.

FIG. 6 illustrates the objects and message pathways for a single transform implementing a single computational operation. When subscriptions are entered, the mechanism of the present invention builds a structure containing all of the transform objects and view objects needed for all intermediate and subscribed views of all subscriptions. This structure is called a dataflow hypergraph. The dataflow hypergraph has nodes corresponding to each view object and hyperedges, which may possibly have more than one input feeding an output, representing each transform object associated with an operation in the subscription specification.

The view objects and transform objects are then allocated to actual brokers in the overlay network, either manually by an administrator or automatically via a service, such as the one described in U.S. patent application Ser. No. 10/841,297, entitled “CONTINUOUS FEEDBACK-CONTROLLED DEPLOYMENT OF MESSAGE TRANSFORMS IN A DISTRIBUTED MESSAGING SYSTEM,” and filed on an even date herewith, status abandoned. The published streams and the subscribed views may be constrained to be located on brokers where the publishers and subscribers actually connect. The placement of the intermediate transform objects and view objects is not constrained. That is intermediate transform objects and view objects may be placed wherever suitable, taking into consideration the capacities of the broker machines and the links, as well as the desired performance. After such allocation of objects to brokers, the result is a distributed transform graph.

FIG. 7 illustrates an example dataflow hypergraph distributed over multiple brokers in accordance with a preferred embodiment of the present invention. In the depicted example, the physical network consists of brokers 710, 720, 730, and 740. There are three publishing clients 702, 704, and 706, and one subscribing client 750. The publishing clients are publishing to three separate published message streams: “buys” 722 on broker 720, “sells” 734 on broker 730, and “matches” 712 on broker 710. The subscribing client 750 subscribes to derived view 748 on broker 740.

Broker 710 also includes transforms 714 and 716, which feed change messages to brokers 720 and 730, respectively. Broker 720 includes view objects 724, 726 and transform objects 725, 727. As an example, view object 726 represents an intermediate derived view or relation, which is based on transform 725, published stream 722, and view 724. Broker 730 includes views 732 and 736, in addition to published stream 734, and also includes transforms 735, 737. Broker 740 includes views 742, 744, 748, and transform 746. View 748 is a subscriber view for subscriber 750. As stated above, multiple publisher clients may provide messages for a single message stream, and multiple subscriber clients may subscribe and receive updates from the same view.

As shown in FIG. 7, the transform graph consists of multiple transform and view objects distributed over all brokers. The paths between objects will sometimes lie within a broker, as is the case between transform object 725 and intermediate view object 726. In other cases, such as the path between transform 727 and intermediate view object 742 (shown with a dotted line), the path must cross over an inter-broker link. It is clear to those versed in the art that the within-broker communications between objects may use cheaper communications means, such as parameter passing between objects, whereas inter-broker communications requires generating physical messages or packets that will cross the link. In a preferred embodiment, the protocols of all view objects and transform objects will be able to recover from lost, out-of-order, or duplicate messages, and, therefore, will work correctly regardless of which paths between objects cross broker boundaries and which do not.

In order to support stateful subscriptions, a history of states is stored in a data storage device. For example, messages from the “matches” published stream 712 are stored in storage 782, messages from the “buys” published stream 722 are stored in storage 784, and messages from the “sells” published stream 734 are stored in storage 786. Storage 782, 784, 786 may be a portion of system memory or may be a persistent storage, such as a hard drive, or a combination thereof. In a system guaranteeing reliable service, published messages will be logged to persistent storage before being propagated. Other states, such as views 742, 744, are preferentially stored in main memory and are not required to be stored persistently.

FIG. 8 depicts a process for deploying transform objects and view objects when a dataflow specification is a declarative specification in accordance with a preferred embodiment of the present invention. A declarative specification is a program, such as middleware program source 802. Program source 802 may be written in a language similar to the SQL syntax, for example. Program source 802 may be compiled using compiler 804 into a dataflow hypergraph. Compiler 804 includes particular algorithms for compiling relational algebraic operators into tailored object code that implements transform objects specialized to incrementally evaluate operators, such as join, select, aggregate, etc., for relations of known data types and key signatures. The object code may be, for example, Java™ programming language code. Compiler 804 represents values of derived states as monotonic knowledge, that is, values in a monotonic domain. A monotonic domain means a set of values that can change value only in one direction. The compiler uses each algebraic expression and the domain of its inputs to determine how to represent the monotonic domain of its result. Because the different values of any field belong to a monotonic domain, the system can always detect which of two values of a single data field is older. As another example, the system distinguishes internally between a “missing value,” which is missing because its value is not yet known (and may later change), and one that is missing because it is known to be absent (and will not change). As yet another example, the system distinguishes between a field having a value that is currently ten, but which may get larger later, from a field whose value is currently ten, but is final and will never change.

Compiler 804 generates an object for each relation and an object for each transform. Each algebraic operation with n operands is associated with one transform object with n input interfaces and one output interface to a view object. For example, the join of SELLS and a second relation are compiled into the transform object 735 and the view object 736. The compiler may then generates a script that is executed at deployment time to instantiate all transform and view objects, connecting an instance of the output interface from each relation to an instance of the input to each transform that uses the relation as an input. Base relations are fed from an “input transform” object, which delivers published messages to the relation. The objects form the knowledge flow graph, or hypergraph, in which published messages enter at base relations and each view object passes the changes to its state to the transform objects that use that relation as an operand. These transform objects then compute the incremental change to the relations to which they are associated and pass that information down to other view objects. Information flows down the graph until it reaches relations associated with subscriptions and then the messages are delivered to “output transforms,” which convert the state change information into messages to subscribing clients.

Each relational operator, such as join, project, group-by, select, etc., is associated with a template used by the compiler to produce an incremental transform. As discussed above, a transform has one or more relations feeding inputs to it and a single relation receiving its output. The incremental transform is an object that, given a message saying that one component of its input has changed, computes the appropriate changes to its output and invokes a method in the output relation indicating what has changed.

The hypergraph may be placed manually by an administrator, or, alternatively, the hypergraph may be optimized using transform placement service 806 and automatically deployed using deployment service 808. The hypergraph is optimized and deployed to broker network 824 by passing broker instructions 812 to broker network 824 and receiving performance information 814 from broker network 824. An optimization step consolidates multiple subscriptions to exploit common computations and performs other simplifications to reduce the total computational load on the system. A placement step allocates the transform objects of the knowledge flow graph to particular brokers for the purpose of load balancing, reduction of bandwidth, and overall reduction of delays between publishers and subscribers.

Broker network 824 receives the knowledge flow graph at deployment time. At execution time, publishers 822 publish message streams to broker network 824. The brokers receive events and propagate messages representing view changes towards the subscribers. Subscribers 828 may then receive messages from subscriber views in broker network 824.

FIG. 9 is a flowchart illustrating the configuration and deployment of a stateful publish-subscribe system in accordance with a preferred embodiment of the present invention. The process begins and receives middleware programming source (block 902). The process compiles the middleware programming source into a knowledge flow graph (block 904). Then, the process receives performance information from the broker network (block 906) and optimizes the knowledge flow graph based upon the broker network performance information (block 908). Thereafter, the process deploys the transform objects and the view objects in the knowledge flow graph to brokers (block 910) and the process ends.

FIG. 10 is a flowchart illustrating the operation of a stateful publish-subscribe system at runtime in accordance with a preferred embodiment of the present invention. The process begins and publishing clients publish messages to publisher hosting brokers (block 1002). Transform objects calculate changes in views based on changes in base relations and/or changes in intermediate views (block 1004). The transform objects generate view change messages that propagate toward the subscriber clients. Then, subscriber clients receive view change messages from subscriber hosting brokers representing the subscribed views (block 1006). The process shown in FIG. 10 repeats as long as messages continue to be published and the publish-subscribe system is operational.

The stateful publish-subscribe system of the present invention is able to be deployed on a wide-area, distributed overlay network of broker machines, communicating by message passing, and being subjected to the possibility of duplicate and out-of-order messages between links. The distributed messaging system of the present invention allows subscriptions expressed as relational algebraic expressions on published message histories. Relational algebra may include, in particular, operators such as select, project, join, extend, group-by, sum, count, and average, for example. The relational algebraic expressions may be mapped to form various query languages, such as SQL and XQUERY. Furthermore, the messaging system of the present invention may allow service specifications that are deterministic and “eventually consistent,” meaning that multiple identical subscriptions eventually receive the same result, but weaker, and hence cheaper to implement, than fully consistent database systems.

It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular data processing system.

The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

What is claimed is:
 1. A method for delivering content, the method comprising: receiving at least one specification for a transform; receiving a plurality of input messages associated with input streams from one or more publishers; applying, by a processor, the transform to the plurality of input messages to form an output message, wherein the output message depends on at least two different input messages from the plurality of input messages, and wherein the output message is based on a derived view that is defined by relational expressions for the at least two different input messages; wherein the output message updates a subscribed state requested by a subscriber; wherein the subscribed state is defined using a declarative specification; wherein the declarative specification includes a relational algebraic specification; and delivering the output message to a subscriber.
 2. The method of claim 1, further comprising: compiling the declarative specification into a hypergraph of incremental transform and view objects.
 3. The method of claim 2, further comprising: deploying the hypergraph on a network of broker machines.
 4. The method of claim 1, further comprising: storing the plurality of input messages in a stable storage.
 5. The method of claim 1, wherein the transform includes an incremental evaluator for one of a set of common relational algebraic operations.
 6. The method of claim 5, wherein the set of common relational algebraic operations includes at least one of sum, count, rain, max, select, project, group-by, and join.
 7. The method of claim 1, wherein receiving the plurality of input messages includes receiving a tuple from a database as an input message.
 8. An apparatus for implementing a stateful subscription service, the apparatus comprising: an overlay network including a plurality of broker machines; a plurality of publishing clients that publish messages to published message streams; a deployment service that deploys a hypergraph defining transform objects and view objects to the plurality of broker machines, wherein at least one transform object defined by the hypergraph has at least one message delivered to its output stream that depends on more than one input messages from an input stream of the transform object, wherein each of the more than one input messages is different and is associated with the published message streams; and a plurality of execution engines running on the plurality of broker machines, wherein each execution engine within the plurality of execution engine receives input messages, processes the input messages using the transform objects, and routes output messages toward subscribers; wherein the at least one message delivered to it's output stream is based on a derived view that is defined by relational expressions for at least two different input messages; wherein the at least message delivered to its output stream updates a subscribed state requested by a subscriber; wherein the subscribed state is defined using a declarative specification; wherein the declarative specification includes a relational algebraic specification.
 9. A data processing system comprising: a system bus; a communications system connected to the system bus; a memory connected to the system bus; and a processor connected to the system bus, wherein the processor executes a set of instructions via the memory to perform actions comprising: receiving at least one specification for a transform; receiving a plurality of input messages associated with one or more input streams from one or more publishers; applying the transform to the plurality of input messages to form an output message, wherein the output message depends on at least two different input messages from the plurality of input messages, and wherein the output message is based on a derived view that is defined by relational expressions for the at least two different input messages; wherein the output message updates a subscribed state requested by a subscriber; wherein the subscribed state is defined using a declarative specification; wherein the declarative specification includes a relational algebraic specification; and delivering the output message to a subscriber.
 10. The data processing system of claim 9, wherein the actions further comprise: compiling the declarative specification into a hypergraph of incremental transform and view objects.
 11. The data processing system of claim 10, wherein the actions further comprise: deploying the hypergraph on a network of broker machines.
 12. The data processing system of claim 9, storing the plurality of input messages in a stable storage.
 13. The data processing system of claim 9, wherein the transform includes an incremental evaluator for one of a set of common relational algebraic operations.
 14. The data processing system of claim 13, wherein the set of common relational algebraic operations includes at least one of sum, count, rain, max, select, project, group-by, and join.
 15. The data processing system of claim 9, wherein the for receiving the plurality of input messages includes for receiving a database tuple from a fixed database as an input message.
 16. A computer program product, for delivering content, the computer program product comprising: a computer recordable-type medium having instructions embodied therewith, the instructions comprising: instructions for receiving at least a specification for a transform; instructions for receiving a plurality of input messages associated with an input stream from one or more publishers; instructions for applying the transform to the plurality of input messages to form an output message, wherein the output message depends on at least two different input messages from the plurality of input messages, and wherein the output message is based on a derived view that is defined by relational expressions for the at least two different input messages; wherein the output message updates a subscribed state requested by a subscriber; wherein the subscribed state is defined using a declarative specification; wherein the declarative specification includes a relational algebraic specification; and instructions for delivering the output message to a subscriber.
 17. A method of delivering a message stream through a network of brokers, the method comprising: compiling a declarative specification into a plurality of transform objects and view creating a dataflow hypergraph based on the compiled declarative specification, wherein nodes of the dataflow hypergraph represent view objects and edges of the dataflow hypergraph represent transform objects, each transform object being associated with a common relational algebraic operation in the compiled declarative specification; optimizing the dataflow hypergraph based on performance information received from a broker network; automatically deploying the optimized dataflow hypergraph to the broker network, the automatically deploying comprising allocating view objects and transform objects to brokers in the broker network; and publishing the message stream through the optimized dataflow hypergraph to a subscriber, the subscriber being connected to the broker network, the publishing comprising: delivering the message stream to the transform objects associated with a table of events in a particular message stream in the optimized dataflow hypergraph, the transform objects computing an incremental change to the associated table of events; propagating an information about the incremental change to view objects in the optimized dataflow hypergraph until the information reaches the table of events associated with a subscription; and responsive to the information reaching the subscription, converting the information about the incremental change into the message stream for the subscriber. 